A DC/DC converter converts a DC voltage having a first value into a DC voltage having a second value different than the first value. In particular, chopped supply converters limit the current consumption to the value necessary to generate a desired value of voltage. FIG. 1 shows a DC/DC converter according to the prior art, which is known as a BUCK converter.
A BUCK converter has a transformer forming a resonant element. The transformer includes a primary winding L1 series-connected with a switch SW between two input terminals, E1 and E2 respectively. It also has a secondary winding L2 of which a first end is connected through a diode D1 to a first output terminal S1 of the converter. The cathode of the diode D1 is connected to the terminal S1. The second end of the winding L2 is connected to a second output terminal S2 of the converter.
The primary circuit of the converter is formed by the loop BP, as understood according to Kirchoff's laws, and includes the primary winding L1 and the switch SW, as well as the arms between the input terminals E1 and E2 of the converter. A voltage source (not shown) delivers a DC input voltage VE between the terminals E1 and E2.
The secondary circuit is formed by a loop BS including the secondary winding L2, the diode D1, as well as the arms connected between the output terminals S1 and S2. One of the arms includes an accumulation capacitor C2. The capacitor C2 and the secondary winding L2 form a resonant circuit. Another arm includes the load, symbolized by a resistor RC, which is parallel-connected to the accumulation capacitor C2 between the terminals S1 and S2.
The output voltage VS delivered to the terminals of the capacitor C2 and applied to the load RC is related to the input voltage VE by the relationship VS=k+VE, where k is the conversion ratio of the converter. The cycles for the opening and the closing of the switch SW are controlled by a management unit UC. The secondary winding L2 stores the energy during the closing cycles and restores it, through the diode D1 to the capacitor C2 during the opening cycle. The flow of a load current through the load RC tends to discharge the accumulation capacitor C2. In other words, the opening and closing cycles of the switch SW chop the input voltage VE at a specified frequency called a chopping frequency. The voltage source delivering the voltage VE does not let through current unless the switch SW is closed. The efficiency of a converter of this kind is therefore very high.
The management unit UC generally delivers a pulse-width modulated signal to control the switch SW. The closing time of the switch SW per time unit or per period is dictated by the management unit UC which determines the value of the conversion ratio k. The management unit may be a microcontroller or a similar device. A device of this kind requires a supply circuit producing a low supply voltage of 5 volts, for example, for its operation.
The supply circuit includes a voltage source distinct from the one delivering the input voltage VE.
Any circuit including a converter of this kind must therefore have two supply sources. A first source generates the input voltage VE, and a second source generates the supply voltage of the management unit UC. This constraint places significant penalties with regards to manufacturing costs and the size of simple electronic circuits including a converter of this kind.
There are known circuits for generating a DC supply voltage of 5 volts from an input voltage VE. However, it is necessary to compensate for the variations of VE if this voltage is not maintained at a constant value. In other words, there is need for a regulation of the value of the voltage generated. These circuits are therefore complex. Furthermore, if the value of VE is significantly greater than the value of the supply voltage of the management unit UC, these circuits may have low efficiency. For example, if the supply voltage of the management unit UC is generated from VE by a drop in voltage across the terminals of a resistive element, significant heat is generated.